so ive been trying to work AO-73 for the past few weekends and i feel like im missing something fundamental. i can hear the downlink fine on 145.950 region, signals sound decent when the bird is high enough, but i cannot for the life of me find my own uplink in the passband. using a FT-818 for the 2m uplink and an SDR on 70cm for monitoring the downlink on a separate laptop.

i know about the doppler thing and ive been using Gpredict to get the shift values, applying them manually which is probably part of the problem honestly. when i key up i dont hear anything on the downlink, not even a squeal or anything to indicate im getting into it. the pass is usually around 12-15 degrees max elevation from my location so maybe thats killing me too. running about 3w into a pair of crossed yagis, Az/El capable.

is there a trick to initially finding yourself in there that im not getting? i read about tuning around a bit while transmitting but that seems like itd just walk over everyone else in the passband which doesnt seem cool

the low elevation passes are definitely making it harder, AO-73 needs a pretty clean shot especially if youre not running much power. 3w should technically be enough if your antenna is tracking well but 12 degrees through trees or rooftops is brutal.

the trick i use for finding myself in the passband -- and yeah it feels weird at first -- is to transmit a short unmodulated carrier (just key up with no audio) while sweeping the downlink in like 1-2 khz steps. you want to do this during a higher pass when the bird is actually workable. dont do it during a crowded pass at peak time, maybe try a midweek morning pass when fewer people are on. once you hear your carrier come back you know where you sit and then you just work backwards to figure out your doppler offset error and correct it. after a couple passes you kinda internalize it.

also gpredict is great but are you feeding it current keps? old elements can throw the doppler calc off enough to matter, especially near AOS and LOS when the rate of change is fastest.

yeah low passes are rough, i had the same issue for a while. one thing that helped me was just running the SDR panoramic display so i could see the whole passband at once, you can actually watch the doppler slide in real time and it makes it way more obvious when youre actually hitting it vs just guessing. if youre already doing that and still not seeing your signal then maybe the uplink antenna pointing is off? i chased a problem like that for like three weekends and it turned out my elevation readout was like 8 degrees off because of how i had the sensor mounted, totally embarrassing but there it is